Field emitter structure and fabrication process providing passageways for venting of outgassed materials from active electronic area

ABSTRACT

Outgassed materials liberated in spaces between pointed field emitter tips and an electrode structure during electrical operation of a field emitter device are vented through passageways to a pump or gettering material provided in a separate space. The passageways may include channels formed through an insulating layer between a base for the field emitters, and the electrode structure, with the channels interconnecting adjacent spaces in a row direction. Where the electrode structure includes a gate electrode layer and an anode layer, similar channels may be formed through an insulator layer provided therebetween. The field emitters may be formed in an arrangement of rows and columns, with the spacing between the columns smaller than the spacing between the rows. Holes are formed by anisotropic etching through the anode, gate electrode, and insulator layers down to the base. Subsequent isotropic etching of the insulator layers through the holes in the anode and gate electrode layers is controlled to cause sufficient undercutting in the insulator layers that adjacent holes merge together only in the row direction to form the channels.

This is a division of application Ser. No. 552,643, filed July 16, 1990.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to field emitter arrays, andmore particularly to a field emitter structure and fabrication processwhich provide venting of outgassed materials from the active electronicarea of the structure.

2. Description of the Related Art

Field emitter arrays typically include a metal/insulator/metal filmsandwich with a cellular array of holes through the upper metal andinsulator layers, leaving the edges of the upper metal layer (whichserves as an accelerator or gate electrode) effectively exposed to theupper surface of the lower metal layer (which serves as an emitterelectrode). A plurality of conically-shaped electron emitter elementsare mounted on the lower metal layer and extend upwardly therefrom suchthat their respective tips are located in respective holes in the uppermetal layer. If appropriate voltages are applied between the emitterelectrode, accelerator electrode, and an anode located above theaccelerator electrode, electrons are caused to flow from the respectivecone tips to the anode.

This structure is comparable to a triode vacuum tube, providingamplification of a signal applied to the accelerator or gate electrode,and operates best when the space in which the electrodes are mounted isevacuated. The three electrode configuration is known as a fieldemitting triode or "fetrode". However, numerous other applications forfield emitter arrays have been proposed, including extremely highresolution flat panel television displays. A major advantage of thefield emitter array concept is that the arrays can be formed byconventional photolithographic techniques used in the fabrication ofintegrated microelectronic circuits. This enables field emitter elementsto be formed with submicron spacing, using process steps integrated withthe formation of signal processing and other microelectronic circuitryon a single chip. A general presentation of field emitter arrays isfound in an article entitled "The Comeback of the Vacuum Tube: WillSemiconductor Versions Supplement Transistors?", by Skidmore,Semiconductor International Industry News, pp. 15-18 (Aug. 1988).

A problem which has remained in conventional field emitter arraystructures involves the liberation of outgassed materials in the activeelectronic area of the device. During operation, electrons ejected fromthe field emitter tips strike the anode material, knocking off molecularparticles of trapped gaseous and solid impurity materials. Thisoutgassing effect creates a plasma or ionization in the spaces betweenthe emitter tips and the anode, which seriously degrades the vacuum inthe spaces and may cause arcing which can lead to destruction of thedevice.

SUMMARY OF THE INVENTION

The present invention overcomes the problems created by the liberationof outgassed materials in the active electronic areas of a field emitterstructure by providing passageways which enable removal of the materialsfrom the active areas for collection. The present invention furtherprovides a process for fabricating a field emitter structure includingventing passageways which are advantageously arranged to facilitateefficient removal of the outgassed materials from the active areas.

In accordance with the present invention, outgassed materials liberatedin spaces between pointed field emitter tips and an electrode structureduring electrical operation of the device are vented through passagewaysto a pump or gettering material provided in a separate space. Thepassageways may include channels formed through an insulating layerbetween a base for the field emitters, and the electrode structure, withthe channels interconnecting adjacent spaces in a row direction. Wherethe electrode structure includes a gate electrode layer and an anodelayer, similar channels may be formed through an insulator layerprovided therebetween. The field emitters may be formed in anarrangement of rows and columns, with the spacing between the columnssmaller than the spacing between the rows. Holes are formed byanisotropic etching through the anode, gate electrode, and insulatorlayers down to the base. Subsequent isotropic etching of the insulatorlayers through the holes in the anode and gate electrode layers iscontrolled to cause sufficient undercutting in the insulator layers thatadjacent holes merge together only in the row direction to form thechannels.

The field emitter structure may further include a structurallysupporting open mesh screen adhered to the opposite side of the base.The base may be formed with at least one hole therethrough whichconstitutes part of the passageways, and which may be covered with themesh screen.

These and other features and advantages of the present invention will beapparent to those skilled in the art from the following detaileddescription, taken together with the accompanying drawings, in whichlike reference numerals refer to like parts.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified plan view illustrating an arrangement of fieldemitters formed on a base in accordance with the present invention;

FIG. 2 is a section taken on a line II--II of FIG. 1, but illustrating acomplete field emitter structure embodying the invention;

FIG. 3 is similar to FIG. 2, but is taken on a line III--III of FIG. 1;

FIG. 4 is a fragmentary perspective view of the present field emitterstructure;

FIG. 5 is similar to FIG. 2, but shows a modified embodiment of thepresent structure; and

FIGS. 6a to 6d are sectional views illustrating a process forfabricating a field emitter structure in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIGS. 1 to 4 of the drawing, a field emitter structureor device embodying the present invention is generally designated as 10,and includes an electrically conductive base 12 made of, for example, ametal or polycrystalline silicon material. A plurality of pointed fieldemitters 14 upstand from a surface 12a of the base 12, and have pointedtips 14a. The field emitters 14 are made of an electrically conductivematerial such as molybdenum or polycrystalline silicon, and are in ohmicconnection with the base 10. The field emitters 14 may be coated with alow work function material such as titanium carbide, which facilitateselectron emission from the tips of the field emitters.

Field emitter arrays have been heretofore formed by two processes, thefirst of which is described in an article entitled "PHYSICAL PROPERTIESOF THIN-FILM FIELD-EMISSION CATHODES WITH MOLYBDENUM CONES", by C. A.Spindt et al, Journal of Applied Physics, vol. 47, No. 12, pp. 5248-5263(Dec. 1976). The main steps of the process include depositing aninsulator layer and a metal gate electrode layer on a silicon substrate,and forming holes through these layers down to the substrate. Molybdenumis deposited onto the substrate through the holes by electron beamevaporation from a small source. The size of the holes progressivelydecreases due to condensation of molybdenum on their peripheries. A conegrows inside each hole as the molybdenum vapor condenses on a smallerarea, limited by the decreasing size of the aperture, and terminates ina point which constitutes an efficient source of electrons.

The second method of fabricating a field emitter array is disclosed inU.S. Pat. No. 4,307,507, issued Dec. 29, 1981, entitled "METHOD OFMANUFACTURING A FIELD-EMISSION CATHODE STRUCTURE", to H. Gray et al. Inthis method, a substrate of single crystal material is selectivelymasked such that the unmasked areas define islands on the underlyingsubstrate. The single crystal material under the unmasked areas isorientation-dependent etched to form an array of holes whose sidesintersect at a crystallographically sharp point. Following removal ofthe mask, the substrate is covered with a thick layer of materialcapable of emitting electrons which extends above the substrate surfaceand fills the holes. Thereafter, the material of the substrateunderneath the layer of electron-emitting material is etched to expose aplurality of sharp field-emitter tips.

The field emitters 14 are shown as having a pyramidal shape as formed inaccordance with the process disclosed by Gray et al. alternatively, thefield emitters 14 may have a conical shape as formed in accordance withthe article to Spindt et al.

Although only eight field emitters 14 are shown in the drawing forclarity of illustration, in an actual device a large number of fieldemitters will be formed on a base and electrically operated in parallelto provide a useful magnitude of electrical current. The field emitters14 are formed on the base 12 in an arrangement of horizontal rows andvertical columns. In accordance with an important feature of a preferredfabrication method of the invention, the spacing between adjacent fieldemitters 14 in the column direction (horizontal spacing between columns)is smaller than the spacing between adjacent field emitters in the rowdirection (vertical spacing between rows).

Further illustrated in FIG. 1 are holes in the shape of elongated slots12b formed through the base 12 between the field emitters 14 and therespective edges of the base 12. An open mesh screen 16 may beoptionally adhered to an opposite surface 12c of the base 12, as visiblein FIGS. 2 and 3, to provide support for the base 12 during fabricationand operation of the device. The screen 16 may preferably be made of ametal such as molybdenum or copper, and be in ohmic connection with thebase 12 and field emitters 14.

Electrically insulative support members in the form of upstanding walls18 are formed on the surface 12a between adjacent rows of field emitters14, as illustrated in broken line in FIG. 1. The walls 18 definechannels 20 therebetween, in which the rows of field emitters 14 arelocated respectively.

A gate electrode layer 22 made of, for example, an electricallyconductive metal such as gold, is supported above the surface 12a by thewalls 18. The electrode layer 22 has holes 22a formed therethrough,aligned above the tips 14a of the respective field emitters 14. Theholes 22a constitute at least part of respective open spaces 24 providedbetween the tips 14a of the field emitters 14 and the edges of the holes22a of the electrode layer 22. The open spaces 24 merge together and arethereby interconnected in the row direction of the structure 10 toconstitute the channels 20.

Electrically insulative supporting walls 26, which are essentiallysimilar to the walls 18, are formed on the electrode layer 22, andsupport an anode layer 28 thereon. The anode layer 28 may be formed ofan electrically conductive metal such as gold. Holes 28a are formedthrough the anode layer 28, in alignment with the holes 22a and fieldemitters 14. If desired, an optional electrically conductive cover layer30 may be adhered to the anode layer 28 in ohmic connection therewith.The walls 26 define channels 32 therebetween which are aligned over thechannels 20.

The structure 10 further includes an enclosure or container 34 in whichthe base 12 and elements formed thereon are mounted. The container 34may be made of any suitable material, and includes a base 36 and a cover38. Although not shown, leads may be provided for connection of the base12, gate electrode layer 22, and anode layer 28 to an external circuit.The container 34 is preferably evacuated, and hermetically sealed.

During operation of the structure 10, an electrical potential which ispositive with respect to the base 12 is applied to the anode layer 28.With a positive potential above a predetermined cutoff value applied tothe gate electrode layer 22, electrons will be emitted from the tips 14aof the field emitters 14 and be accelerated to the anode layer 28. Theconductive cover layer 30, if provided, constitutes an integral anodestructure in combination with the anode layer 28. The magnitude ofelectron flow depends on the potential applied to the gate electrodelayer 22. Increasing the gate electrode potential produces an increasein the anode current, with a gain or amplification factor inherent inthe configuration enabling the structure 10 to function as an amplifierin a triode configuration.

The electrons emitted from the field emitters 14 strike the anode layer28 and cover layer 30 with sufficient energy to cause outgassing orliberation of trapped gaseous and solid impurity materials into theactive electronic areas between the field emitter tips 14a and the anodelayer 28. Unless removed, the outgassed materials may cause sufficientionization or plasma formation in these areas to cause seriousmalfunction or destruction of the device as discussed above.

In accordance with an important feature of the present invention, thechannels 20 and 32 constitute at least part of a network of passagewayswhich enable venting or removal of the outgassed material from theelectronically active areas to a separate area in which a pump, orgettering means, which functions as a pump, is provided for collectionof the materials. As best seen in FIG. 2, a gettering material 40 suchas barium, which acts as a concentration gradient driven pump, is coatedon the upper and side walls of the interior of the cover 38. Theoutgassed materials in the active electronic areas below the holes 28ain the anode layer 28, due to their initial high concentration in theseareas, are pumped or diffuse through the channels 20 and 32 to theexternally located gettering material 40 which traps the materials. Theventing and collection process continues as long as a concentrationgradient exists between the active electronic areas, and the areas onwhich the gettering material 40 is formed.

In addition to the inner walls of the cover 38, the gettering material40 may be formed on the inner surface of the base 36 of the container34, below the mesh screen 16. Outgassed materials will be additionallyvented from the channels 20 and 32, through the holes 12b formed throughthe base 12, and the mesh screen 16, to the gettering material 40 on thebase 12.

These venting paths or passageways may be provided singly, or in anydesired combination. It is further within the scope of the invention toreplace the gettering material with an external pumping means, whichcommunicates with the channels 20 and 32 through a hole (not shown)formed through the container 34. As a yet further modification of thepumping means, most materials, with the notable exception of elementswith completely filled atomic shells, are chemically reactive inatomically pure form. By making the inner walls, or at least part of theinner walls, of the container 34 extremely clean or atomically pure, theatomically pure surfaces will exhibit a gettering effect in a mannersimilar to the material 40.

FIG. 5 illustrates a modified field emitter structure 10' embodying thepresent invention, in which like elements are designated by the samereference numerals and corresponding but modified elements aredesignated by the same reference numerals primed. The structure 10'differs from the structure 10 in the provision of holes or slots 42,which are formed through the base 12' by plasma etching or the like, andcommunicate directly with the spaces 24. The slots 42 enable venting ofoutgassed materials therethrough from the spaces 24 to the getteringmaterial 40 provided on the base 36, and may be provided in addition to,or as an alternative to the channels 20. Where the slots 42 are providedwithout the channels 20 and 32, they constitute passageways incombination with the open mesh screen 16 which interconnect the openspaces 24.

FIGS. 6a to 6b illustrate a preferred process for fabricating the fieldemitter structure 10 in accordance with the present invention. In FIG.6a, the field emitters 14 are formed on the base 12 using a processdisclosed in the references discussed above, or any other process whichwill produce an equivalent result. In FIG. 6b, an electricallyinsulative layer 50 of, for example, silicon dioxide, is formed over thebase 12 to cover the field emitters 14. A conductive metal layer 52 of,for example, gold, is formed over the insulative layer 50. A secondinsulative layer 54 is formed over the conductive layer 52, and a secondconductive layer 56 is formed over the insulative layer 54.

In the step illustrated in FIG. 6c, a layer 58 of a photoresist materialsuch as Shippley AZ 1370 photoresist is formed over the conductive layer56 using a photolithographic technique employing a mask (not shown),which leaves holes 58a through the layer 58 aligned over the fieldemitters 14. An etching process which is substantially anisotropic, suchas plasma etching employing a substance that does not etch thephotoresist layer 58, is used to etch substantially vertical holes 56a,54a, 52a, and 50a through the layers 56, 54, 52, and 50 respectively.Following this step, the photoresist layer 58 may be removed.

As illustrated in FIG. 6d, an etching process which is at leastpartially isotropic, such as wet etching employing a material such asCF₄, NF₃, or SF₂, that does not etch the conductive layers 52 and 56, isused to etch the insulative layers 50 and 54. In accordance with animportant feature of the present invention, the etching step illustratedin FIG. 6d is controlled such that the holes 50a and 54a in theinsulative layers 50 and 54 are expanded to undercut the holes 52a and56a in the conductive layers 12 and 56 to an extent such that adjacentholes 50a and 54a merge together only in the row direction of thestructure 10 to form the channels 20 and 32 respectively. This occursbecause the spacing between the field emitters 14 in the columndirection is smaller than the spacing in the row direction. An equalamount of etching in both directions will cause adjacent holes 50a and54a to merge together in the row direction, but not in the columndirection, due to the larger spacing between the holes in the rowdirection. In FIG. 6d, the layers and holes which have been modified bythe isotropic etching step are designated by the same reference numeralsprimed The layers 50, 52, 54, and 56, and the holes formed therethrough,correspond to the elements 18, 22, 26, and 28 illustrated in FIGS. 1 to4 respectively.

While several illustrative embodiments of the invention have been shownand described, numerous variations and alternate embodiments will occurto those skilled in the art, without departing from the spirit and scopeof the invention. Accordingly, it is intended that the present inventionnot be limited solely to the specifically described illustrativeembodiments. Various modifications are contemplated and can be madewithout departing from the spirit and scope of the invention as definedby the appended claims.

We claim:
 1. A process for fabricating a field emitter structure, comprising the steps of:(a) forming a plurality of upstanding, electrically conductive, pointed field emitters on a surface of an electrically conductive base in an arrangement of rows and columns such that the spacing between adjacent columns is smaller than the spacing between adjacent rows; and (b) forming electrode means supported above said surface, portions of the electrode means supported above said surface, portions of the electrode means adjacent to the points of the field emitters being separated therefrom by open spaces respectively, and passageway means which interconnect said open spaces; step (b) including the substeps of: (c) forming an electrically insulative layer on said surface covering the field emitters; (d) forming the electrode means as an electrically conductive layer on the insulative layer; (e) forming holes through the conductive layer aligned with the points of the field emitters respectively; and (f) forming holes in the insulative layer through said holes in the conductive layer respectively; said holes in the insulative layer exposing the points of the field emitters and constituting at least part of said open spaces in combination with said holes in the conductive layer respectively; said holes in the insulative layer being formed such as to undercut said holes in the conductive layer sufficiently to merge together only between adjacent columns and form channels which constitute at least part of the passageway means.
 2. A process as in claim 1, in which steps (e) and (f) in combination comprise the substeps of:(g) forming a resist layer on the conductive layer having holes aligned with the points of the field emitters respectively; and (h) substantially anisotropically etching the conductive layer and insulative layer through said holes in the resist layer using a substance that does not etch the resist layer; step (f) further including the substep, performed after step (h), of: (i) at least partially isotropically etching the insulative layer through said holes in the conductive layer using a substance that does not etch the conductive layer.
 3. A process as in claim 2, further comprising the step, performed after step (h), of:(j) removing the resist layer from the conductive layer.
 4. A process for fabricating a field emitter structure, comprising the steps of:(a) forming a plurality of upstanding, electrically conductive, pointed field emitters on a surface of an electrically conductive base in an arrangement of rows and columns such that the spacing between adjacent columns is smaller than the spacing between adjacent rows; (b) forming a first electrically insulative layer on said surface covering the field emitters; (c) forming an electrically conductive electrode layer on the first insulative layer; (d) forming a second electrically insulative layer on the electrode layer; (e) forming an electrically conductive anode layer on the second insulative layer; (f) forming a resist layer on the anode layer having holes aligned with the points of the field emitters respectively; (g) substantially anisotropically etching the anode layer, second insulative layer, electrode layer, and first insulative layer through the holes in the resist layer using a substance that does not etch the resist layer; and (h) at least partially isotropically etching the second and first insulative layers through the holes in the anode and electrode layers using a substance that does not etch the anode and electrode layers, such that the holes in the second and first insulative layers undercut the holes in the anode and electrode layers respectively sufficiently to merge together only between adjacent columns to form channels.
 5. A process as in claim 4, further comprising the step, performed after step (g), of:(i) removing the resist layer.
 6. A process as in claim 4, further comprising the step, performed after step (h), of:(i) adhering an electrically conductive layer to the anode layer in electrical connection therewith. 